Multiple genetic marked strains of Chinese hamster ovary cells (CHO) have been isolated. These strains are Pro minus Pur minus Amin R. Other mutant markers will be introduced into these stocks. These mutants will be studied in the following manner and for the following characteristics. 1) Purine mutants will be studied by analyzing the biochemical block, testing for CRM ion/CRM minus protein, and possibly enzyme purification and characterization. 2) Analysis of genetic lesions by complementation, chromosome mapping, and karyotyping. 3) Isolation of temperature sensitive mutants in this and other strains. 4) Study of the regulatory effects of purine and amino acid deprivation on eukaryotic cells. 5) Effect of host cell mutation on subsequent viral infection. 6) "Transduction type" phenomenon resulting in gene transfer by extracellular induced agents, or DNA (or RN) tumor viruses.